Higher cellulose esters



Patented July 12, 1949 Robert D'IRowley; *NarrowsrVa iassi'gnor toiGelanese Corporat'ion of Americagaf'corporation of ,1 Delaware No Drawing. Apiilication 'septeniber 11, 1945, Serial No. 6115;693

"2 Claims.

This invention relates to the production'of'or- .ganic acidiestersoi cellulose, and relates'more particularly to the production of cellulose triesters which are highly stable and which may be precipitated intasoft and'openfibrous form* so that they'may be easily washedfor otherwise treated.

An objectof this invention is the provisionbf i pmved process for the productionof stable organic acid esters of' cellulose of -a higher degree Ofesterification from organic-acid esters of cellulose of a lower degree of esterification.

Another object of thisinvention is the preparationofcellulose"trkacetate of improved stability and other physical characteristics by a process comprising the acetylation of cellulose "acetates of a lower'degreeof acetylation.

-A further objector this invention isthe' preparation of organic acid esters of cellulose of high degree of esterification; including mixed organic =acidesters of cellulose, without the :use of catalysts suchas sulfuric acid or the liketoaid the esterification.

Other objects of this invention wil1 appear I from the following detailed description.

In the process of preparing organicacid esters of cellulose; such as cellulose acetate, for exam- .ple, the esterification reaction isusually carried out by treating cellulose with anorganicacidan- 'hydridein' the presenceof a catalyst such as sulfuric-acid and'an organic acid: diluent; 'or solvent, for the cellulose ester being formed. The esteri- 'fied cellulose is obtained in the'formof a viscous, homogeneous solution in the organic aciddiluent.

Water is then a'ddedin anamount sufficient to i convert any organic acid anhydride remaining-to the corresponding acid. The cellulose ester, -u'suany afterthe addition of a further quan'tity of water, is permitted to hydrolyze" or ripen frdm a cellulose 'tri-ester to a cellulose ester of a lower degree ofesterification having the *desiredsolu- *bility characteristics. L During ripening not only are acyl groups hydrolyzed oif 'but'in addition combined sulfuric acid is removed. *"Waterand/or other non-solvent for the 'cellulose" ester is then added in sufiicient amounts to precipitate the cellulose-ester from solution. =The' precipitated cellulose-ester is washed with water to remove as much'acid and" other non-cellulose ster mate- =rials as possible and l is then subjected to "a stabilizing treatment with the obJ'ectof still further reducing its content of combined sulfuric acid. 'Any 'combined sulfuric acid imparts to thecellu- "lose ester *a tendenc to-ridecompose, degrade vmaid/ordiscolor. The' 'degreebf stability' is -meas- -ured by the degree"of acidity developed when' a "sampleof the cellulose ester is' treated with distilled water at elevated temperatureand pressure fora predetermined period of time. The develop- *ment or excess acidity denotes'a product of unsatisfactory stability.

Fully ester'ified or unhydrolyzed esters of cellulose;such'asS'Tor example, cellulose tri-acetate, 1 are"usua3lly iii an unsati'sfactory degree "of stability. 'This' is probably due to the fact that the 'esters have not undergone any hydrolysis and, "thereforejlittle or none of the combined sulfuric acid has been split 61f. Furthermore, stabilization of these fully esterified cellulose esters does not have the desired 'fiect since on precipitation from the esterify-ing medium theiully esterified. cellulose estersno'rmallyprecipitatein a hardand "hornyiorm instead of asa loose, open fiber. The

hard an'd"horny'precipitated cellulose esters are not only diflicult towash but, in addition, strongly resist penetration by liquids, making any attempt "atstabilization quite ineffective. Since the combined sulfuric acidcannot be removed, such' fully ester'ified cellulose esters are not very stable and undergo rapid '-discoloration, degradation or de- 'composition,particularly when exposed to elevatd temperatures as during molding operations.

I-have found that fully esterified organic acid esters "of cellulose of an excellent degree of stability andwhich' maybe precipitatedwith a soft, "loose,-open' fibrous structure amenable to washing and drying may now be obtained. In accordance with my novel" process, said fully esterified "organic acid esters of cellulose may be obtained by dissolving a hydrolyzed organic acid ester of cellulose, prepared in -the usual manner well known in the art, in a suitable catalyst-free solventhm'edium =and then subjecting the cellulose ester while in "solution to 're-esterification' at elejvated' temperature employing as the esterifyin'g agent any 'desired organic acid anhydride. The reaction mixture is maintained at the elevated -t'emperature until the "desired degree of esterification takes place. The cellulose ester formed "may then'be precipitated from solution in fibrous "formafter e'x-cess anhydrideis'des'troyed, by the addition-'ofiwater thereto.

While my'n'ovel process isof particular value for the production of fully esterified organic acid esters: of cellulose and fully es'terified mixed or- -ganic'"-acid esters of cellulose, itis to be understood' that' 'the re esterification may, of course, be haltrd"--at any" point short of the formation of the fullyeSterified cellulose tri=ester. In this than the initial, hydrolyzed cellulose esters from which they are prepared by the re-esterificationprocess described.

Thus, for example, in the preparation of cellulose acetates of a higher degree of acetylation, e. g. 57 to 62.5% acetyl value, calculated as acetic acid, from hydrolyzed cellulose acetates of a lower degree of acetylation, the cellulose acetate employed is dissolved in from 3 to 10 parts, based on the weight of the cellulose acetate of glacial acetic acid after which 0.1 to 2.0 parts by weight of acetic anhydride are added thereto. The amount of acetic anhydride added depends upon the degree of acetylation desired in the reesterified cellulose acetate formed.

Without the addition of any esterification catalyst thereto, the reaction mixture may be heated to a temperature of 60 to 100 C., or more, for 6 to 200 hours to eifect the desired re-acetylation. Depending upon the amount of acetic anhydride added to the reaction mixture and the time during which the reaction mixture is maintained at elevated temperature, cellulose acetates of any degree of acetylation up to and including the tri-acetate may be formed. The progress of the re-acetylation may be followed very closely by the usual solvent and chloroform tests, the solubility or lack of solubility in the various solvents or mixtures of solvents employed indicating the degree of acetylation which has taken place. The higher cellulose acetates thus formed may then be precipitated from solution by the addition of a suitableamount of water thereto. The cellulose acetates precipitate in the form of a soft, loose, white fiber, regardless of the degree of acetylation which has taken place. The precipitated cellulose acetates may be washed and dried quite readily. Higher cellulose acetates substantially free of combined sulfuric acid and of a very high order of stability are obtained by my process.

In order further to illustrate my invention, but without being limited thereto, the following examples are given:

, Example I 10 parts by weight of hydrolyzed cellulose acetate of 49% acetyl value, calculated as acetic acid, are dissolved in 40 parts by weight of glacial acetic acid. To the solution obtained is added parts by weight of acetic anhydride. The anhydride added comprises an amount sufficient to reacteylate this cellulose acetate to an acetyl value of 54.4%, plus a excess. No catalyst is added. The reaction mixture is heated to 70 C. and maintained at this temperature for 24 hours. The re-acetylated cellulose acetate formed is precipitated from solution by the addition of an excess of water thereto. The loose, open fiber obtained is then washed neutral. On analysis, the cellulose acetate is found to have an acetyl value of 54.9%, calculated as acetic acid. The cellulose acetate is considerably more stable than the cellulose acetate from which it is prepared by re-acetylation, developing only the acidity of the original cellulose acetate when heated in distilled water at elevated temperature and pressure.

Example II 10 parts by weight of hydrolyzed cellulose acetate of an acetyl value of 54.4%, calculated as acetic acid, are dissolved in 50 parts by weight of glacial acetic acid and 5 parts by weight of acetic anhydride are added thereto, the anhydride being sufiicient to form a cellulose triacetate on reaction with the hydrolyzed cellulose acetate. No catalyst is present in the reaction medium. The latter is then heated to C. and held at this temperature for 48 hours.

At the end of the first 24 hours. the cellulose acetate has an acetyl value of 61.7%, calculated as acetic acid, and at the end of 48 hours is fully esterified to the tri-acetate, having an acetyl value of 62.5%. Thecellulose tri-acetate is precipitated from solution by the addition of an excess of water thereto and the cellulose triacetate precipitates in the form of a very soft, loose, open white fiber that may be easily washed and dried. The cellulose tri-acetate obtained is soluble in cold chloroform and insoluble in acetone. On being heated with distilled water at elevated temperature and pressure the cellulose tri-acetate develops very little acidity, i. c. has a high order of stability.

While my invention has been more particularly described in connection with the preparation of improved cellulose acetates of a higher degree of acetylation, including cellulose triacetates, my novel process may, as stated, also be employed for the preparation of other organic acid esters of cellulose of a higher degree of esterification from organic acid esters of cellulose of a lower degree of esterification. Examples of other improved organic acid esters of a higher degree of esterification which may be obtained by my novel process are cellulose propionate, cellulose tri-propionate, cellulose butyrate and cellulose tri-butyrate as well as mixed esters of a higher degree of esterification and fully esterified mixed esters such as cellulose acetatepropionate, cellulose acetate-butyrate, cellulose acetate-stearate, cellulose acetate-laurate, cellulose acetate-crotonate and cellulose butyratecrotonate.

It is to be understood that the foregoing detailed description is given merely by Way of illustration and that many variations may be made therein without departing from the spirit of my invention.

Having described my invention, what I desire t-e secure by Letters Patent is:

1. A process for the production of cellulose ri-acetate from hydrolyzed cellulose acetate,

which consists of dissolving a hydrolyzed cellulose acetate of an acetyl value of about 54.4%, calculated as acetic acid in glacial acetic acid, adding acetic anhydride to the solution obtained in an amount at least sufficient to form the desired tri-acetate, heating the reaction mixture produced to an esterifying temperature, and maintaining said mixture at the esterifying temperature for 48 hours until the cellulose acetate is fully esterified.

2. Process for the production of cellulose triacetate from hydrolyzed cellulose acetate, which consists of dissolving a hydrolyzed cellulose ace;- tate of an acetyl value of about 54.4%, calculated as acetic acid in glacial acetic acid, adding sumcient acetic anhydride to the solution obtained file of this patent:

UNITED STATES PATENTS to form the desired tri-acetate, heating the re- Number Name Date action mixture produced to a temperature of 1,668,945 Clarke et a1. May 8, 1928 60 to 100 C., and maintaining said mixture at 5 1,668,946 Clarke et al. May 8, 1928 the esterifying temperature for 48 hours until 1,999,406 Dreyfuss et al. Apr. 30, 1935 the-cellulose acetate is fully esterified. 2,053,280 Fothergill Sept. 8, 19.36

, ROBERT D. ROWLEY. FOREIGN PATENTS REFERENCES CITED m Number Country Date The following referenlces are of record in the 3393324 Germany Aug. 12, 1921 

